Shared mail from deactivated users

We run into this issue every time someone in key positions leave the company: We want to close the mail account, but his replacement should have access to their old mailbox. Currently it seems that all user states other than Active simply disables shares. What I'm looking for is some kind of attic where I can send users that quit such that they're not listed in the admin's normal user list (to keep some semblance of tidiness) and GAL, can't log in, won't receive new mails but returns a custom auto-reply message, and, crucially, existing shares are still maintained.